The use of filled polypropylene compositions is expanding in the fields of exterior and interior automotive trims, in electrical equipment device housings and covers, as well as for other consumer product applications. The materials are popular, at least in part, because they often provide a pleasant tactile sensation.
Filled polypropylene compositions are commonly prepared by melt compounding a polypropylene, a filler, and optional additional components. This usually involves one or more pre-press compounding operations. Filled polypropylene compositions may also be prepared by a processing approach referred to as “at-press” compounding which includes feeding separate particles of a masterbatch composition containing a high concentration of filler and particles of a polypropylene into a part fabrication equipment (e.g., a molding machine) in which the particles are melted, blended, and then shaped into a part. Descriptions of at-press compounding of filled thermoplastics are disclosed for example in U.S. Patent Application No. 61/036,692, filed on Mar. 14, 2008 (now International Patent Application Publication No. WO 2009/114761, published on Sep. 17, 2009), U.S. Patent Application Publication No. US 2007/0246862 A1 (filed on Apr. 17, 2007), and US 2008/0058455 A1 (filed Jun. 25, 2007), all of which are incorporated by reference herein.
Articles molded from filled polypropylene compositions are prone to exhibit surface imperfections, such as flow marks and silver streaks.
Flow marks may manifest on the surface of injected molded articles as a series of alternating high and low gloss bands or stripes, sometimes giving rise to the term “tiger striping”. The general trend of each band is approximately perpendicular to the direction of melt flow during injection. These marks do not perceivably affect mechanical properties of the molded article, nor are they discernible by touch. However, their presence is aesthetically unacceptable and often results in unacceptably high quality control reject rate because of the appearance of inhomogeneity within molded parts. The effect is pronounced in large molded articles with a high aspect ratio, such as automobile parts, for example, instrument panels and bumper fascia.
The art has attempted to improve surface appearance properties in molded articles by decreasing the viscosity of the propylene polymer resin. This technique decreased the appearance of the flow marks, however the decrease in viscosity may detrimentally effect other physical properties such as impact strength. Flow marks may also be decreased by annealing the article after the molding process. This annealing step however requires increased dependence on energy to anneal the article, as well as additional time to anneal, and possible equipment modification necessary to allow injection-molding apparati to also serve as annealing mediums. The art has also described adding low viscosity rubber components to polypropylene to improve the appearance of the resultant injection-molded articles, see for example U.S. Pat. No. 5,468,808. Expensive additives, such as fluoropolymers have also been used to improve the appearance of filled TPO compositions as described in PCT Patent Application Publication No. WO2007/024541A2, published Mar. 1, 2007, incorporated by reference herein.
Notwithstanding many successful efforts to date, there exists an ongoing need to provide additional processing techniques to help reduce the flow marks in articles produced from propylene polymer compositions without compromising other physical properties, such as, impact strength.
There also continues to be a need for additional filled TPO compositions that can be used in applications requiring molded parts having good appearance (e.g., automotive interior). For example, there continues to be a need for compositions which result in parts which are free of or have reduced tiger-striping; which exhibit an advantageous balance of high ductility, high impact strength, and/or low stiffness; which exhibit good appearance (e.g., when molded with a molded-in-color); and which are free of, or substantially free of a fluoropolymer).